The goals of this research are to test the hypothesis that the small scaffold protein MP1 is required for ER function and proliferation of ER- positive breast cancer cells and to characterize the ER/MP1 complex. MP1 expression was inhibited in several breast cancer cell lines by transfection with MP1-targeting siRNAs. The results obtained demonstrate that MP1 is required for the survival of ER-positive MCF-7 cells but not ER-negative MDA-MB-231 cells. This suggests that the requirement for MP1 may be specific to ER-positive cells in which case it could provide a novel target for treatment of this class of breast tumor. To facilitate studying the ER/MP1 complex a Flag-MP1 gene has been cloned and reconstituted into a retrovirus vector. Cells infected with this virus efficiently express the Flag-MP1 gene for at least 9 days and stable cell lines containing this construct are currently being selected for use in characterizing a novel ER/MP1 complex.